Two stars in the constellation Corona Borealis, the northern crown, are like a celestial yin and yang. One star is normally bright enough to just see with the unaided eye, but periodically disappears. The other is normally far too faint to see, but periodically flares to naked-eye visibility.

The disappearing star is R Corona Borealis. It’s many times larger and heavier than the Sun, and thousands of times brighter. Under dark skies, it’s barely visible even though it’s thousands of light-years away.

But the star sometimes grows so faint that you need a good-sized telescope to see it. That’s because it expels atoms of carbon, which link up to form a cloud of carbon molecules — tiny particles like the soot from a chimney. The veil soon disappears, though, and R Corona Borealis returns to view.

It’s just the opposite for T Corona Borealis — a binary system that’s far too faint to see without a telescope. But the smaller member of the system — a white dwarf — steals hydrogen from a bloated companion star. When enough hydrogen piles up on the white dwarf, it triggers a thermonuclear blast, making the system shine thousands of times brighter.

Two outbursts have been observed. The system will almost certainly erupt again, although no one is sure just when. That makes T Corona Borealis a system to keep an eye on — even if we can’t actually see it.

Look for the northern crown high in the east at sunset and standing high overhead around midnight.